[RFC] Implement new feature: Custom messages

[saulvaldelvira]

This PR adds an option to provide a custom function to write the message.

I've also fixed a small typo in one of the tests.

I haven't added any documentation yet. If the maintainers like the idea, I'll gladly document this.

Related issue: #54

[epage]

In our contributing documentation, we ask for problems and solutions to be worked out in issues and to leave PRs for code review.

[saulvaldelvira]

In our contributing documentation, we ask for problems and solutions to be worked out in issues and to leave PRs for code review.

Hello! Sorry for that. Should I add an issue to discuss this, and link to the PR?

[epage]

You could post you use case and idea to #54, it seems related enough.